\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath,amsfonts}
\usepackage{color}
\pagestyle{empty}
\begin{document}
\begin{equation*}
  C_i(s) = k \frac{s+a}{s+b}, \qquad a = 2,\quad b = 50,\quad k =
  1.
\end{equation*}
\end{document}